1. Field of the Invention
The present invention relates to an image sensor for detecting rays and a method of manufacturing the same.
2. Description of the Related Art
Image sensors for detecting rays such as hard X rays or γ rays and generating image data are used in various fields of technology. The image sensor may detect a radiation field emanating from a celestial body, which can be analyzed to determine the physical conditions and spatial structure of the celestial body. The image sensor may apply X rays to a human subject or the like and detect the waves passed through it, from which a tomogram of the human subject or the like can be obtained. The image sensor is used in other fields. For example, it is employed in nuclear engineering (e.g., examination of glass-sealed radioactive waste and ray-monitoring), nondestructive inspection (e.g., inspection of semiconductor devices), and a resources survey (e.g., a survey of resources in the ground).
The image sensor hitherto used in these fields of art has, for example, the structure described below.
FIG. 1A shows a conventional image sensor 80 of a representative type. As shown in FIG. 1A, the image sensor 80 has a detecting element (Si element or Ge element) 81 and an amplifier IC 84. The detecting element 81 detects the incident rays and generates electric signals from the rays. The amplifier IC 84 lies in the same plane as the detecting element 81. It amplifies the electric signal the element 81 has generated. Bonding wires 83 connect the output wires of the detecting element 81 to the amplifier IC 84 by bonding wires 83.
FIG. 1B is a cross-sectional view, taken along line 1B—1B shown in FIG. 1A. As FIG. 1B shows, the detecting element 81 has a semiconductor (Si, Ge, CdTe, CdZnTe, or the like) layer 87 and electrodes 85 and 86. The electrodes 85 and 86 hold the layer 87 between them. A voltage is applied between the electrodes 85 and 86, thereby detecting rays of various energy bands, from the electrons and protons generated as rays are applied to the image sensor 80.
An image sensor that employs an Si element or a Ge element can detect only low-energy X rays of few keV to tens of keV. Its sensitivity is far from the value that is demanded in practice.
Even a high-sensitivity image sensor using CdTe or CdZnTe cannot efficiently collect electric charges for the following reason, so long as its structure remains conventional. The semiconductor layer may be made thick in order to promote the scattering of high-energy particles in the material. If the layer is thick, the distance between the electrodes will increase. This lowers the efficiency of collecting electric charges. Consequently, the sensitivity will decrease. To enhance the efficiency of collecting electric charges, the semiconductor layer may be made thin. In this case, the scattering of high-energy particles in the material will be suppressed. The sensitivity of the image sensor will inevitably decrease, though the charge-collecting efficiency increases.
The present invention has been made in consideration of the foregoing. An object of the invention is to provide an image sensor that can detect rays, even those of hard X-ray energy band, with high sensitivity, and to provide a method of manufacturing such an image sensor.
Another object of the present invention is to provide a hole-making technique in handling harmful substances, such as CdTe, which impose no adverse influence on human beings.